Botulinum toxin (BoTx) has long been known as one of the most potent inhibitors of neuromuscular transmission (by blocking the release of acetylcholine from nerve endings) and has been used to treat several conditions where spasm of skeletal muscle is felt be an important contributory factor. However, until now there had been no attempt to explore the use of this unique biological agent in the treatment of disorders of gastrointestinal muscle or other disorders of smooth muscle.
The muscle in the gastrointestinal tract differs from muscle elsewhere in two major ways. First, most of the muscle in the gastrointestinal tract is of a type called smooth muscle. There are several fundamental differences between the way smooth muscle and skeletal muscle function. First, smooth muscle lacks a discrete end-plate (a defined region of interaction between the nerve ending and muscle, as seen in skeletal muscle); instead nerve fibers run from each axon parallel to the muscle bundle and end somewhat arbitrarily at various points along its length. Secondly, unlike skeletal muscle, smooth muscle cells are coupled electrically within large bundles by means of connecting bridges. An electrical event at any region in the bundle is therefore conducted in a decremental fashion to other regions. Thirdly, each muscle bundle receives input from multiple axons in the form of either excitatory or inhibitory signals (see below). This is in contrast to skeletal muscle outside the gastrointestinal tract, where typically only one type of neurotransmitter is operative.
In addition, the gastrointestinal muscle is organized and regulated very differently than muscle elsewhere. Both skeletal and smooth muscle in the gastrointestinal tract are under the control of the enteric nervous system which is an extremely complex network of nerves and muscles, that resides within the gastrointestinal wall and orchestrates the entire digestive process including motility, secretion and absorption. The enteric nerves are also organized into interconnected networks called plexuses. Of these, the myenteric plexus, situated between the circular and longitudinal muscle layers, is the main modulator of gastrointestinal motility. It receives input from both the central nervous system (via vagal and sympathetic pathways) as well as from local reflex pathways. Its output consists of both inhibitory and excitatory signals to the adjacent muscle.
The final neural pathway regulating muscle activity in the gastrointestinal tract is therefore represented by the neurons of the myenteric plexus. A useful, if somewhat simplistic concept is to visualize net muscle tone in the gastrointestinal tract as that resulting from the balance between the opposing effects of two neuronal systems in the myenteric plexus: one causing the muscle to contract (mainly via acetylcholine) and the other causing it to relax. Both types of neurons, however, are activated by acetylcholine within the myenteric plexus. The role of acetylcholine in the regulation of gastrointestinal muscle tone is therefore complex. Acetylcholine directly released by effector nerves near the muscle causes contraction; however, within the royenteric plexus, it may result in inhibition or excitation. This is in contrast to skeletal muscle outside the gastrointestinal tract which is directly innervated by nerves emanating from the central nervous system. The interaction between nerve and muscle in skeletal muscle outside the gastrointestinal tract is far more simple: nerves release acetylcholine which causes the muscle to contract.
Finally, the myenteric plexus is probably the most important but not the only determinant of muscle tone in the gastrointestinal tract. In fact, basal smooth muscle tone may be visualized as resulting from the sum of many different factors including intrinsic (myogenic) tone, and circulating hormones, in addition to nerve activity.
It should be clear therefore, that the regulation of gastrointestinal tract muscle motility is far more complex than that of skeletal muscle outside the gastrointestinal tract. While there have been isolated reports on the effects of botulinum toxin on in vitro preparations of gastrointestinal smooth muscle, the regulation of gastrointestinal muscle is so complex that the physiological consequences of blocking neurotransmitter release (by using toxin such as botulinum) in humans or in live animals were not predictable prior to the present invention.
There is a need in the medical arts for methods and devices for treatment of gastrointestinal disorders including achalasia, other disorders of the lower esophageal sphincter, sphincter of Oddi dysfunction, irritable bowel syndrome, etc., which treatments will be long-lasting and devoid of significant rates of complication.